1. Field of the Invention
The invention relates generally to seafloor magnetic measurements, and, more particularly to a method and system for correcting magnetic based orientation measurements.
2. Background Art
The present invention is particularly related to remotely operable electromagnetic (EM) measurement systems, such as Magnetotelluric (MT) or controlled source electromagnetic (CSEM) measurement systems. Both MT and CSEM use similar receiver setups. They differ in the sources of EM energies used in the surveys. MT uses natural EM perturbation induced in the formation. CSEM uses a controlled EM source that is towed by a ship, typically at about 50 feet from the seafloor, where the receiver arrays are placed. Because the receivers used for MT and CSEM share similar properties, the following description will focus on MT measurement. However, one of ordinary skill in the art will appreciate that the description is equally applicable to CSEM measurements, or other EM measurement techniques such as IP, TDEM or hybrid seismoelectric techniques.
MT measurements are used to compute an EM impedance of selected earth formations. MT measurements are especially useful in regions where seismic imaging is inappropriate. For example, MT exploration is useful when evaluating geologic formations such as salts and carbonates. Salts, carbonates, and other particular formations may scatter seismic energy when seismic energy is propagated through them because of large velocity contrasts and inhomogeneties located within these formations, whereas the EM energy of the MT source fields propagates through these layers with less distortion. MT methods measure variations in the earth's magnetic and electric fields, and do not rely on seismic energy to determine formation characteristics.
MT methods may be used to measure an EM impedance as a function of frequency. Lower frequency provides a greater depth of penetration. The measured impedance may be transformed into an apparent resistivity and/or conductivity of the selected formations. Measuring impedance at several locations at various frequencies enables a determination of resistivity and/or conductivity as a function of both depth and horizontal position. Therefore, MT methods may be used to evaluate formation resistivity over large areas of the seafloor. The formation resistivities of the various formations in a selected area may then be analyzed to determine the formation geometry, the presence or absence of hydrocarbons in selected formations, and the like.
MT methods are passive methods, in that MT methods use natural variations in the earth's magnetic field as an energy source. Such methods include a subsea system that detects orthogonal magnetic and electric fields proximate the seafloor to define a surface impedance. The surface impedance, as described above, may be measured over a broad range of frequencies and over a large area where layered formations act in a manner analogous to segments of an electrical transmission line. One MT method that operates according to the principles described above is generally disclosed in U.S. Pat. No. 5,770,945 issued to Constable. The type of EM receiver disclosed therein can also be used to record EM signals that originate from various kinds of transmitter systems, such as a towed cable bipole or magnetic loop source.
In addition, the receivers could be used to detect EM radiation originating from other types of signals such as emanating from naval ships (corrosion currents, electric circuits, generators, moving machinery) or from electric or magnetic sources located in boreholes or nearly land sources. The objective of these measurements could range from detailed exploration of the subsurface conductivity structure to monitoring naval traffic or operations to determining leakage signals from subsea cables.
Referring to FIG. 1, a subsea system that may be used in MT methods includes an apparatus such as an MT measurement system 100 disclosed in the Constable patent. The MT measurement system 100 includes a body 102 having a battery pack (not shown), a data acquisition system 104, two orthogonally oriented magnetic sensors 122 and 124, and four arms 139, 140, 142, and 144, each of which includes an electrode 118, 119, 120, 121 mounted at the end thereof. The electrodes 118, 119, 120, 121 are silver-silver chloride electrodes, and the magnetic sensors 122, 124 are magnetic induction coil sensors.
The arms 139, 140, 142, 144 are five meters long and approximately 2 inches in diameter. The arms 139, 140, 142, 144 may be formed from a semi-rigid plastic material (e.g., polyvinyl chloride or polypropylene) and are fixed to the body. The arms 139, 140, 142, 144 are designed to rest on the seafloor when the MT system 100 is deployed.
The body 102 is attached to a releasable concrete anchor 128 that enables the MT system 100 sink to the seafloor after deployment. The body 102 generally rests on top of the anchor 128 when it is positioned on the seafloor. The anchor 128 may be released after MT measurements have been completed so that the body 102 may rise to the surface and be retrieved by a surface vessel (not shown).
In seafloor EM measurements, the three-dimensional (3D) orientation of the sensors is determined from independent measurements of the static magnetic field of the Earth H and measurements of the acceleration of gravity, g. An orientation measurement instrument uses a magneto-resistive device, fluxgate magnetometer, or DC magnetometer to measure the three components of the magnetic field and a three-component tilt meter to measure g. Alignment on the horizontal plane (heading) is obtained from the measure of the horizontal components of H, while the pitch and roll angles are obtained from measurements of g. Note the dip could alternatively be determined by directly reading a tiltmeter device such as a pendulum or electrolytic bubble tiltmeter.
The measurement of the Earth's static magnetic field may be biased due to the presence of locally originated magnetic fields. Sources of locally originated magnetic fields may include induction sensors having highly permeable cores, remnant magnetic fields in the steel casing surrounding a battery, acoustic transducers, and fields arising from current flow within the electronics. As a result of locally originated magnetic fields, the total magnetic field measurement is a superposition of the Earth's static magnetic field with the biasing fields arising from local sources. The effect of the biasing fields cannot be uniquely calibrated for a given data acquisition system because the biasing fields change with each deployment from modifications to the MT measurement system, including a new battery, induction sensors in a different position, or any other changes involving ferrous metal or electricity. Thus, calibration is required for each deployment of a MT measurement system.
In the related art, there are several calibration/orientation techniques. One technique is known as a sugar cube compass. In this technique, a compass needle floats on a solution that freezes at low temperatures. When the solution freezes, the compass needle is locked at the orientation of the sensors. Local magnetic sources can bias the compass reading. In some instances, the freezing point of the solution may not be reached at the seafloor, keeping the compass needle from being locked in place.
Another related art calibration/orientation technique is data correlation with natural fields. Recorded EM data is correlated with data acquired at a reference site of known orientation. For example, the reference site may be on land. This technique is supported by the fact that plane wave natural fields are assumed homogeneous across both the land reference and seafloor deployments of the data acquisition system. Accuracy of the technique depends largely on data quality and may be unable to be used if one of the horizontal magnetic field measurements is missing, or distorted by subsurface structures.
Another related art calibration/orientation technique is data correlation with controlled source fields. In controlled source EM surveys, a transmitter antenna supplies a signal to be analyzed. The orientation of the transmitter antenna is assumed to be known, and then the data can be rotated to maximize the amplitude at the closest position of the transmitter. This technique relies on exact knowledge of the transmitter antenna position and orientation which change as a transmitter dipole is towed.